1. Field of the Invention
This invention relates to a method of manufacturing an optical element such as a color filter for operating as a component of a color liquid crystal device to be used typically for a color television, a personal computer, etc. or an electroluminescence element having a plurality of light-emitting layers for full color display by utilizing an ink-jet system.
2. Related Background Art
The demand for liquid crystal displays, color liquid crystal displays in particular, has been increasing in recent years, keeping pace with the technological advancement in the field of personal computers including portable personal computers. However, to further boost the demand, the cost of color liquid crystal displays must be reduced further particularly in terms of the color filters they comprise, because the color filters account for a significant portion of the overall manufacturing cost.
While various techniques have been proposed to date in an attempt to meet the above requirement and also the requirements for improving color filter performance, no satisfactory solution has been found so far. Known methods for preparing color filters will be summarily discussed below.
First, there is a dyeing method. With a dyeing method, a layer of a water-soluble polymer material is formed as a dyeing layer on a transparent substrate and subjected to a patterning operation using photolithography to produce a desired pattern, which is then immersed in a dyeing bath to obtain a colored pattern. The above sequence of operation is repeated three times to produce a colored layer comprising differently colored sections of three colors of R (red), G (green) and B (blue).
Second, there is a pigment dispersion method, for which massive research efforts have been paid in recent years. With a pigment dispersion method, a photosensitive resin layer containing a pigment in a dispersed state is formed on a transparent substrate and then subjected to a patterning operation to obtain a single color pattern. The above sequence of operation is repeated three times to produce a colored layer comprising differently colored sections of R, G and B.
Third, there is an electrodeposition method. With this method, a transparent electrode formed on a transparent substrate is patterned and immersed in an electrodeposition painting solution containing a pigment, resin and electrolytic liquid to electrodeposit a first color. This process is repeated three times to produce a colored layer comprising differently colored sections of R, G and B, which is then baked.
With a fourth method, a pigment is dispersed in thermosetting-type resin and printed. This process is repeated three times using three different colors of R, G and B, and subsequently the resin is thermally set to produce a colored layer. With any of the above described methods, a protection layer is normally formed on the colored layer.
What is common to all the above-described methods is that a process must be repeated three times for R, G and B, consequently raising the cost. Additionally, any methods involving a large number of steps entail a problematic low yield. Furthermore, in the case of an electrodeposition method, the profile of the pattern that can be formed by electrodeposition is quite limited, and hence the method is hardly applicable to the process of forming a liquid crystal element of the TFT type (to be used with an active matrix drive method using a TFT (thin film transistor) as switching element).
A printing method is accompanied by a problem of poor resolution and hence hardly applicable to the formation of a pattern having a fine pitch.
As an attempt to avoid the above-identified problems, efforts have been made to develop a method of manufacturing color filters that utilizes an ink-jet system. A manufacturing method using an ink-jet system provides an advantage of a simple manufacturing process and low manufacturing cost.
Additionally, an ink-jet system is applicable to manufacturing not only color filters but also electroluminescence elements.
An electroluminescence element comprises a thin film containing a fluorescent organic or inorganic compound that is sandwiched by a cathode and an anode and is adapted to generate excitons when electrons or holes are injected into the thin film for recombination, so that it can be made to emit light by means of the emission of fluorescence or phosphorescence that occurs when the excitons are deactivated. Thus, an electroluminescence element can be formed by applying a fluorescent material to be used for the electroluminescence element onto a substrate carrying TFT elements formed therein to produce a light-emitting layer there.
The ink-jet system finds applications in the manufacture of optical elements including color filters and electroluminescence elements, because it provides an advantage of a simple manufacturing process and low manufacturing cost as pointed out above. However, the manufacture of optical elements using the ink-jet system is accompanied by problems such as “intermingling of colors” and “blank areas” that are specific to the ink-jet system. These problems will be discussed below in terms of manufacturing color filters.
The problem of “intermingling of colors” arises when inks of different colors are intermingled between any two adjacent pixels (colored sections) showing different colors. With a method of manufacturing color filters, using a black matrix of an appropriate material as partition walls and forming colored sections by applying inks to the respective openings of the black matrix, inks need to be applied by a volume several times to tens of several times greater than the capacity of the openings. If the inks contain solid ingredients such as a coloring agent and a hardening component to a high concentration and hence the volume of inks to be applied is relatively small, the black matrix operates satisfactorily as partition walls and can sufficiently retain inks in the openings so that any applied ink would not flow over the black matrix to reach an adjacent colored section showing a color different from that of the ink. However, on the other hand, if the inks contain solid ingredients only to a low concentration and hence a large volume of ink has to be applied, the applied ink would flow over the black matrix and become intermingled with the other inks in adjacently located colored sections. Particularly, since there is a limit to the viscosity of ink that can be ejected stably from the nozzle of an ink-jet head and also to the concentration of the solid ingredients contained in the inks, a special and cumbersome technique is required to avoid the problem of intermingling of colors.
There have been proposed techniques for preventing intermingling of colors by utilizing the wettability of ink between the colored sections and the partition walls. For instance, Japanese Patent Application Laid-Open No. 59-75205 describes a method of forming an anti-diffusion pattern, using a poorly wettable material, in order to prevent ink from flowing into areas other than target areas. However, the above patent document does not specifically teach how to form such a pattern. On the other hand, Japanese Patent Application Laid-Open No. 4-123005 describes a method of forming partition walls for preventing intermingling of different colors by patterning a silicone rubber layer that is highly water-repellent and oil-repellent. Additionally, Japanese Patent Application Laid-Open No. 5-241011 and Japanese Patent Application Laid-Open No. 5-241012 also disclose methods of forming a silicon rubber layer on a black matrix operating as a light-shielding layer so that it can be used as partition walls for the purpose of preventing intermingling of colors.
With any of the above methods, the ink applied to such an extent that it exceeds by far the height of the partition walls is repelled by the ink-repellent surface layer of the partition walls so that the ink does not flow over the partition walls into any adjacent colored sections, and the intermingling of colors can be effectively prevented.
FIGS. 3A and 3B of the accompanying drawings schematically and conceptually illustrate the problem of intermingling of colors that arises with known methods of manufacturing an optical element. Referring to FIGS. 3A and 3B, a black matrix 33 is formed on a transparent substrate 31 and operates as partition walls. In FIGS. 3A and 3B, reference numeral 36 denotes ink. If the upper surface of the black matrix 33 is ink-repellent, the applied ink 36 is retained in the right openings of the black matrix 33 and would not flow into any adjacent colored sections as shown in FIG. 3B. However, if the upper surface of the black matrix 33 is poorly ink-repellent, the applied ink 36 can spread over the black matrix to wet the latter so that it can be intermingled with the ink applied to adjacent openings as shown in FIG. 3A.
Generally, fluorine compounds are more ink-repellent than silicon compounds. For instance, Japanese Patent Application Laid-Open No. 2000-35511 discloses a method of forming a positive-type resist pattern on a light-shielding section and applying an ink-repellent chemical agent onto the pattern. It also discloses the use of a fluorine compound as an ink-repellent chemical agent. However, with the proposed method, the positive-type resist pattern formed on the light-shielding section needs to be removed after forming colored sections, and a problem of dissolution, separation and swelling of pixels can occur when removing the resist pattern.
As a technique of fluorinating the surface of a resin layer, the Journal of the Japan Society of Chemistry, vol. 10, pp. 1916-1923 (1985), proposes a method of treating with reactive gas of a fluorine compound by turning it into plasma. Japanese Patent Application Laid-Open No. 11-271753 and Japanese Patent Application Laid-Open No. 11-32974 disclose a technique of applying the above method to a color filter. According to these patent documents, partition walls are made to show a multi-layer structure of a lower layer having an affinity for ink and an upper layer which is rendered to have no affinity to ink by subjecting it to a plasma-treatment using gas containing a fluorine compound.
However, according to either of the above-cited patent documents, the partition walls must be made to show a multi-layer structure so that a photolithography process must be repeated a plurality of times, thereby making the overall process a complex one, which in turn raises the manufacturing cost and reduces the manufacturing yield.
On the other hand, the problem of “blank areas” mostly arises when the applied ink cannot spread sufficiently and uniformly in the areas surrounded by partition walls and can end up with a defective display effect due to uneven color distribution and poor color contrast.
FIGS. 4A and 4B of the accompanying drawings schematically illustrate blank areas. The members in FIGS. 4A and 4B that are the same as those of FIGS. 3A and 3B are denoted respectively by the same reference numerals. In FIGS. 4A and 4B, reference numeral 38 denotes a blank area.
In recent years, in the technological field of color filters for TFT type liquid crystal elements, the openings of the black matrix 33 are normally made to show a complex profile and have a number of corners in order to protect the TFTs against external light and/or obtain a large aperture ratio and bright displayed images. Then, there arises a problem that the applied ink 36 does not spread satisfactorily to the corners as illustrated in FIG. 4A. Additionally, as a photolithography process involving the use of resist is normally employed to form a black matrix 33, various contaminants contained in the resist can adhere to the surface of the transparent substrate 31 to prevent the applied ink 36 from spreading satisfactorily. Furthermore, if the lateral surfaces of the black matrix 33 are extremely ink-repellent when compared with the surface of the transparent substrate 31, the ink 36 can be repelled by the lateral surfaces of the black matrix 33 in a manner as shown in FIG. 4B, so that the contact areas of the ink 36 and the black matrix 33 can show a faded color.
As an attempt to prevent the problems of intermingling of colors and blank areas, Japanese Patent Application Laid-Open No. 9-203803 proposes the use of a substrate that is processed to show affinity for ink so as to make the (recessed) areas surrounded by the black matrix (projecting sections) show a contact angle of smaller than 20° relative to water. As a method of providing the substrate with affinity for ink, the patent document teaches the use of a water-soluble levelling agent or a water-soluble surface-active agent. The document further discloses a technique of treating the surfaces of the projecting sections preliminarily with an ink-repellent treatment agent to make the surfaces ink-repellent. More specifically, it describes the use of a fluorine-containing silane coupling agent as an ink-repellent treatment agent and that of a fluorine type solvent for the purpose of coating. According to the above patent document, only the top surfaces of the projecting sections are made ink-repellent and the lateral surfaces thereof are not (1) by using layers of two different materials so that the projecting sections per se may have such properties; (2) by covering the areas of the transparent substrate other than the projecting sections with resist and treating only the top surfaces of the projecting sections for ink-repellence; or (3) by forming a resist layer on the transparent substrate, treating the entire surface for ink-repellence and subsequently forming projecting sections by patterning the resist layer by means of a photolithography process.
Japanese Patent Application Laid-Open No. 9-230129 also describes a technique of providing the recessed areas with affinity for ink by irradiating the transparent substrate with energy rays. Again, according to the above patent document, only the top surfaces of the projecting sections are treated for ink-repellence by applying a photosensitive material for forming projections onto a glass substrate, treating the entire surface with an ink-repellent treatment agent and subsequently patterning the photosensitive material in a photolithography process. Thereafter, both the projecting sections and the recessed areas are treated or either the projecting sections or the recessed areas are selectively treated to provide them with affinity for ink by means of irradiation of energy rays.
However, with any of the above described methods, as the surface of the projecting sections is treated for ink-repellence and subsequently the recessed areas are treated so as to be provided with affinity for ink, the ink-repellence of the surface of the projecting sections is reduced during the treatment for affinity for ink. Therefore, it is difficult for the surface of the transparent substrate and the lateral surfaces of the black matrix to be provided with sufficient affinity for ink and, at the same time, for the top surface of the black matrix to be made satisfactorily ink-repellent. Furthermore, while Japanese Patent Application Laid-Open No. 2000-18771 proposes a technique of providing partition walls with ink-repellence by treating them with gas plasma of a fluorine compound, since the treatment for ink-repellence is carried out after a treatment for affinity for ink, ink would not wet the partition walls nor spread in areas where ink is applied to consequently give rise to problematic blank areas.
The above problem equally arises when manufacturing electroluminescence elements by means of an ink-jet system. More specifically, when organic semiconductor materials that emit light in R, G and B, respectively, are used as inks and pixels (light-emitting layers) formed by applying the inks in corresponding areas that are surrounded by partition walls, the light-emitting layers would not emit light in desired color to a desired level of brightness in areas where inks are intermingled between adjacent light-emitting layers. Additionally, when the electroluminescence element is made to have a single light-emitting layer, all the pixel areas surrounded by partition walls are filled with an equal amount of ink. Therefore, if ink flows from a pixel area into an adjacent pixel area, a problem of disparity arises among pixel areas in terms of the amount of ink, which by turn gives rise to an uneven distribution of brightness. Additionally, if ink does not spread satisfactorily in each area surrounded by partition walls, the boundary zones of the light-emitting layer and the partition walls would not provide a sufficient level of brightness of emitted light. In the following description on manufacturing electroluminescence elements, mixing of inks between adjacent light-emitting layers is referred to as “intermingling of colors” and areas along the boundaries of light-emitting layers and partition walls where a problem of disparity arises in terms of brightness of emitted light are referred to as “blank areas” for the purpose of convenience.